Recently, with sophistication of informatization technology, information recording technology, particularly magnetic recording technology is significantly improved. As a substrate for a magnetic recording medium such as HDD (Hard Disk Drive) as one of media for such magnetic recording, an aluminum substrate has been widely used. However, with reduction in size or thickness of a magnetic disk, and with increase in recording density, demand for a glass substrate is now increased, the glass substrate being high in surface flatness and high in strength compared with the aluminum substrate.
Moreover, with increase in recording density in magnetic recording technology, a magnetic head is now changed from a thin film head to a magnetoresistive head (MR head) or a giant magnetoresistive head (GMR head), and flying height of a magnetic head on a substrate is accordingly reduced to about 8 nm. A magnetic head mounted with such a magnetoresistive effect element sometimes induces a thermal asperity trouble as a unique trouble of the head.
The thermal asperity trouble is a trouble where when a magnetic head in flying in the air passes above a small convex or concave portion on a magnetic disk surface, the magnetoresistive effect element is heated due to adiabatic compression of air or contact with the small portion, causing a read error. Therefore, for the magnetic head mounted with the magnetoresistive effect element, a magnetic disk surface is required to have extremely high smoothness and flatness. Moreover, when a magnetic layer is formed on a glass substrate with dust or a foreign substance being adhered, a convex portion is inconveniently formed, therefore the glass substrate is required to be highly cleaned for completely removing the dust or the foreign substance.
Furthermore, substrate size recently tends to be reduced in order to mount a large-capacity magnetic recording medium on a mobile device. Therefore, a 1.8-inch substrate, a 1-inch substrate, or a further small-size substrate is required rather than a 3.5-inch substrate or a 2.5-inch substrate in the past. When a substrate is reduced in size in this way, an allowable dimension error is also reduced, consequently more precise inner-diameter processing is required.
In addition to smoothness and flatness of the magnetic disk surface, strict accuracy control is required for a dimension error in inner diameter of a circular hole provided in a center of a magnetic disk. This is because a dimension error of an inner circumferential end face of a magnetic disk has a direct influence on setting accuracy when the magnetic disk is fittingly set on a spindle motor of HDD. Moreover, a large dimension error in inner diameter leads to a possibility of inducing a mechanical error in stacking servo (writing of servo information into a magnetic disk) performed before the magnetic disk is assembled in a magnetic disk device such as HDD, or a possibility of inducing bad fitting of the disk with a spindle in disk stacking. The inner circumferential end face of a magnetic disk is small in surface area compared with a main surface, and when a rotational center of the magnetic disk is displaced due to the dimension error in inner diameter, it is difficult to dispose a head of HDD in a correct position on the HDD, consequently data are hardly recorded or reproduced.
Moreover, since magnetic disk is subjected to read/write of data while rotating at high speed, it is necessary that data on the magnetic disk do not move even during such high-speed rotation. Therefore, accuracy control of a dimension error in inner diameter is particularly important for a substrate for a magnetic disk.
Furthermore, when attention is paid on data access for HDD, a servo pattern to be an index for positioning is previously written into a magnetic disk assembled in the HDD to accurately store/reproduce data of the magnetic disk. Such writing of the servo pattern is carried out while the magnetic disk is fittingly set on a device called servo writer. The magnetic disk written with the servo pattern is temporarily separated from the servo writer, and fittingly set on the spindle motor of HDD being a product.
In the case that the dimension error in inner diameter of a magnetic disk is large, when the magnetic disk is assembled in HDD, alignment between a servo pattern and a position of a recording/reproducing head of HDD being a product is disordered, therefore recording/reproducing of data is still not normally performed. While a technique of adjusting alignment for correcting such a positional relationship is disclosed, the technique does not give a drastic solution for suppressing the dimension error in inner diameter.
Under such a situation, to avoid the thermal asperity trouble, it is necessary to smoothen a surface of a magnetic disk, in addition, to smoothen (mirror-finish) an end face of the magnetic disk. Moreover, to prevent movement of a rotation axis when the magnetic disk is fitted with the spindle motor, an inner circumferential end face of the magnetic disk needs to be processed at high accuracy. Thus, disclosure is made on a technique of polishing the inner circumferential end face of the magnetic disk into an end face having a certain roughness or lower by using a polishing brush having brush hair curled in a meandering pattern (for example, see Patent Document 1 (JP-A-2004-155652)).
Patent document 1: JP-A-2004-155652